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Scylla
01-20-2000, 04:35 PM
The Tiger lives a while, does Fun Tiger Stuff!! Breeds (if it's lucky,) and eventually dies.

For an Amoeba it's all eat, divide, eat divide, ad nauseum, ad infitum. The amoeba achieves immortality in it's endless monotony since when it divides you can't tell which was the original (or maybe you can, I don't care.)

So, what's to blame?

SEX

If you can't clone yourself, when you die that's it.

Sex has obvious advantages from an evolutionary standpoint (fun too,) but death seems a terrible price to pay for a quickie.

My question is how did this get started?

Did one Amoeba say to another "Hey Babe, I got a great idea c'mere!" or what?

I'm taking a wild guess and thinking that simple unicellular organisms exchanged DNA through Viruses, and this somehow evolved into the backseats and bedrooms we all know and love.

Does anybody know if this is true?
How did sexual reproduction evolve?
Any good references or books where one can "Bone up" on The Straight Dope?



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Often wrong.... NEVER in doubt

Yarster
01-20-2000, 05:08 PM
Any good biology textbook will tell you that sex evolved to provide for genetic diversity. Think about it, if you reproduce an exact copy of yourself, and a nasty virus or environmental condition comes along that wipes you out...hey guess what? That's right, it wipes out all of your clones too.

With sex, there is some hope that you will evolve a gene that will provide a resistence to that bug, or a phenotype that can withstand the environmental hardship, etc. that would otherwise mean extinction of the entire species.

Another consequence of the whole thing is that the need for sex tends to encourage populations to stay somewhat closeby. That is, to 'school'. That affords better protection to the group as a whole in some cases (i.e. fight off large predators) and also makes it harder for predators to find the population as a whole, weeding out their numbers.

Of course, some fish have a great deal where they can change sexes, and I know that some less developed species (insects in some cases) can go either way. That is, sex if there's someone around to get some action from, or parthenogenesis (sp?) if you have to 'go at it alone'.

Scylla
01-20-2000, 05:13 PM
Thank you Yarster. I would like to know how it evolved, not why. I am interested in the evolutionary steps that led to sexual reproduction. Perhaps I should have been clearer.

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Often wrong.... NEVER in doubt

tanstaafl
01-20-2000, 06:08 PM
Well, in general you start with something like an amoeba just drifting around and dividing; absorbing anything it drifts into that is vaguely foodlike and occasionally being absorbed by something bigger than it.

After a while, a couple of amoeba like things divide but don't separate from each other immediately. Since this blob of several amoebas is larger, it gets eaten by fewer bigger things and lasts longer.

Sometime after that some of the amoebas making up the blob start being somewhat different when they divide. Say, the ones on one end develop little bits that they can wiggle and push water with. The whole blob can now move a little on its own and doesn't have to depend on drifting around.

Now that it can move, some other cells happen to develop that can detect food. The organism can now move towards food. It can now become even larger.

This continues up the line with the organism getting more and more complicated.

As for sex itself developing... At some point an organism starts pulling off a part of another organism (looking for food). The pulled off part contains cells; some of which are used as food but others may attach themselves to the organism. The first organism now gets some of the benefits of the other organisms DNA. (Remember, we are talking about very simple organisms here.) After a while, some organisms could evolve that would deliberately attempt to gain DNA cells from other organisms (as opposed to depending on a few to not get digested). Other organisms may evolve to design structures that would be easily torn off. This becomes a benefit to both organisms.

Add a few hundred thousand more steps over a few billion years and you get from your amoeba to your tiger.

Scylla
01-20-2000, 08:30 PM
Tanstaafl:

Thanks. Do you know what the names of these organisms are?

Where can I find info on them?

Subsuming DNA by cannibalization seems unlikely, no odder than viruses exchanging it though, I'll admit.

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Often wrong.... NEVER in doubt

Rysdad
01-20-2000, 10:18 PM
Originally posted by tanstaafl:
At some point an organism starts pulling off a part of another organism

Oh? You knew my ex?

coosa
01-21-2000, 12:24 AM
Scylla, I think you are basically asking a question that doesn't have a definite answer. You're talking about millions of years of evolution, for which we have only a spotty record to date. The original 'critters', such as amoebas, were soft-bodied animals that didn't fossilize very well and didn't leave a lot of evidence behind for scientists to work with. And how do you pick out THE single point at which a certain development occurred? Unless a definite transitional creature just happened to die in such a place and circumstance that it became well-fossilized, and someone who is qualified to recognize that creature just happens to come across it while digging around somewhere, we may never be able to point a finger at a specific example.

Most evolutionary changes were very, very slight differences that appeared and accumulated over millions of years, not sudden major changes that made the huge step from asexual-to-sexual in a single bound.

And it's a little hard to duplicate millions of years of evolution in a laboratory. What we basically have are occasional examples of creatures that appear to be very similar in many ways and significantly different in others, separated by huge blank areas that we attempt to fill in with logical progressions based on what knowledge we have so far.

I'd suggest getting your hands on a good biology text of some kind and do some serious studying on bacterial reproduction. Although bacteria reproduce asexually, they sometimes join together and trade bits of DNA with each other (this is why antibiotic resistance can develop and spread so rapidly). Think of it as a information exchange between creatures that can't read or write. This information exchanged proved so advantageous in some creatures that they developed more and more complicated ways of dealing with their environment, and continued to trade this information with other similar creatures. As the creatures themselves became more complicated, the method of exchanging information became more complicated also. Complicated multicellular individuals couldn't really take advantage of this new information without disrupting the delicate balance that enabled their many cells to function as a unit. However, they could store their own information in a separate unit, and join this information to that of another individual to create offspring that included information from both.

So eggs and sperm developed, and whenever possible two organisms would get together and pool their information by forming a new individual. The new individual would eventually meet another individual of like mind, and they would combine THEIR information into another new individual - etc., etc. Voila! Sexual reproduction at work.

That probably didn't make a lot of sense to anybody but me, but hey, I tried!

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Sacred cows make the best hamburgers. - Mark Twain

moriah
01-21-2000, 01:01 AM
Two million quatloos on the tiger.

tanstaafl
01-21-2000, 09:15 AM
Scylla - No, I don't know that there is a specific named creature for each of the steps. I'm repeating the steps from a biology course I took a long time ago, but I don't think the professor ever named them other than by saying "ok, at the next stage...".

I notice I left off the final stage, which is where the two organisms start coming together to voluntarily exchange genetic material. And thus sex was born.


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"Drink your coffee! Remember, there are people sleeping in China."

Dennis Matheson --- dennis@mountaindiver.com
Hike, Dive, Ski, Climb --- www.mountaindiver.com (http://www.mountaindiver.com)

Jinx
01-21-2000, 09:33 AM
Ever heard of asexual reproduction? Those organisms are not much fun at parties.

Scylla
01-21-2000, 09:37 AM
I was hoping that perhaps there are some bacteria or viruses alive today that demonstrate the evoltionary steps of sexual reproduction.

Thanks for the good help. It's a good starting point for me.

The microbiology text I scanned didn't talk about how it occured. It just mentioned it did.

I'll check out bacteria.

Alphagene
01-21-2000, 04:46 PM
Scylla, check out "sex pili" when you are looking at bacteria. I don't have any references here, but some bacteria can exchange genetic information that way.

Besides that, you're better off looking at protists and invertebrates rather than bacteria if you are wondering about the transition from asexual to sexual.

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Remember when your dog ate my goldfish, and you lied and said I never had a goldfish? Then why did I have the bowl, Bart? Why did I have the bowl?

John Kentzel-Griffin
01-22-2000, 11:11 PM
If I remember Hi School Biology, the paramecium (paramecia?) exchange DNA, but don't have sexes. They also reproduce asexually.

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Virtually yours,

DrMatrix

TheUnforgiven
01-23-2000, 02:06 AM
OK. Coosa and Alphagene have it, I think. Bacteria mostly reproduce asexually via binary fission, which is basically just self replication by dividing itself. They can also sexually reproduce through 1) Transformation, in which foreign DNA can be recombined with the prokaryote's DNA via a transport protien in the cell wall. This is very rare; 2) Conjugation, where a sex pili is formed between two prokaryotes so one cell can receive the plasmid (small chunk of DNA) from the other which then recombines with the original DNA. In 3) Transduction a virus can incorporate foreign DNA into a cell which the cell is then forced to replicate.
I think number 2 is the most likely starting point that later developed into tigers mating.
Mitosis of eukaryotic cells and meiosis of gametes fits in this grand scheme somewhere, too.
Please correct me if I am wrong on anything.

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"The most merciful thing in the world, I think, is the inability of the human mind to correlate all its contents."
-H.P. Lovecraft, "The Call of Cthulhu"

NanoByte
01-23-2000, 05:11 AM
tanstaafl:

Well, in general you start with something like an amoeba just drifting around and dividing; absorbing anything it drifts into that is vaguely foodlike and occasionally being absorbed by something bigger than it.

So what was that bigger thing, and did it get bigger by using sex? Or are we supposed to be starting with amoebae in the first place? And since there seem to be plenty of amoebae around today, why did they bother to make things more complicated?

And in this game of survival of the fittest, what level of organic organization should be seen as that which is to gain by fitness? Why chase the sexual fit?

It seems I've heard that some lower organisms' reproduction can involve more than two "sexes". But the three-body problem seems to be a bear for physicists. And yet biologists do it with protozoa or whatever?

Scylla:

The microbiology text I scanned didn't talk about how it occured. It just mentioned it did.

Ah, you didn't get ahold of the unexpurgated version.

DrM:

If a mecium can't reproduce sexually, why have a paramecium?

Ray (Stay tuned for more stages of devolution.)